As part of PEP-479, an extra check was added to contextlib._GeneratorContextManager to avoid getting confused when a StopIteration exception was raised in the body of the with statement, and hence thrown into the generator body implementing the context manager.

This extra check should only be used when the passed in exception is StopIteration, but that guard is currently missing, so it may unchain arbitrary RuntimeError exceptions if they set their __cause__ to the originally passed in value.

Compare the current contextmanager behaviour:

>>> from contextlib import contextmanager
>>> @contextmanager
... def chain_thrown_exc():
...     try:
...         yield
...     except Exception as exc:
...         raise RuntimeError("Chained!") from exc
... 
>>> with chain_thrown_exc():
...     1/0
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

To the expected inline behaviour:

>>> try:
...     1/0
... except Exception as exc:
...     raise RuntimeError("Chained!") from exc
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Chained!

The __exit__() method doesn't conform PEP-8.

PEP-8: """Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this as return None , and an explicit return statement should be present at the end of the function (if reachable)."""

The __exit__() method has explicit "return False", bare "return", and implicit "return" at the end of the method. Together with different styles in different "except" clauses this makes it slightly hard to read.

PEP-8's rule makes sense elsewhere, but for context managers I think an implicit return None is the norm and that making it explicit wouldn't improve readability.

Then the exception for the __exit__ method should be documented in PEP-8.

New changeset 00c75e9 by Nick Coghlan (svelankar) in branch 'master':
bpo-29692: contextlib.contextmanager may incorrectly unchain RuntimeError (GH-949)
00c75e9

This has been merged for 3.7, but cherry-picks to the other branches are still needed.

I also inadvertently missed adding svelankar's name (Siddharth Velankar) to Misc/ACKS, so that oversight will need to be tidied up as well.

New changeset e8a6bb4 by Nick Coghlan in branch 'master':
bpo-29692: Add missing ACKS entry (bpo-1079)
e8a6bb4

New changeset 9b409ff by Mariatta in branch '3.6':
[3.6] bpo-29692: contextlib.contextmanager may incorrectly unchain RuntimeError (GH-949) (bpo-1105)
9b409ff

New changeset 4d015a4 by Mariatta in branch '3.5':
[3.5] bpo-29692: contextlib.contextmanager may incorrectly unchain RuntimeError (GH-949) (bpo-1107)
4d015a4

PR has been backported into 3.5 and 3.6. Thanks all :)